Generally, liquid crystal has a birefringence characteristic, that is, a refractive index of light in a longitudinal axis direction of molecules of the liquid crystal is different from a refractive index of light in a transverse axis direction of molecules of the liquid crystal. Due to such a birefringence characteristic of the liquid crystal, persons may feel different refractive indexes of light depending on their positions with respect to an LCD. Accordingly, when linearly polarized light passes through the liquid crystal, a polarizing state of the light is changed with various ratios, so an amount of light and the color characteristic perceived by the persons may vary depending on positions of the persons with respect to the LCD. Therefore, a liquid crystal display device having a twisted nematic structure may represent various contrast ratios, color shifts, and a gray inversion phenomenon according to viewing angles.
In order to compensate for a phase difference created in a liquid crystal cell, a TN LCD technique using a phase difference compensation film has been developed. According to the TN LCD technique, the phase difference of light created in the liquid crystal cell is compensated for by means of the phase difference compensation film, so that the above-mentioned problems derived from the viewing angles can be solved. However, a TN LCD using a negative phase difference compensation film can improve a gray characteristic only when a viewing angle forms an angle of 45° with respect to a transmission axis of a polarizing plate, while representing uneven image quality and a halftone gray inversion phenomenon. Meanwhile, a VATN-LCD (vertically aligned TN-LCD) using liquid crystal having a negative dielectric anisotropy property has been developed. The VATN-LCD can be fabricated with low costs and simple processes as compared with the TN-LCD, while representing a superior contrast ratio, low threshold voltage and fast response time.
In a case of the TN-LCD, if voltage is not applied to the liquid crystal, a longitudinal axis of liquid crystal molecules is spirally twisted in parallel to a substrate. In this state, if voltage is applied to the liquid crystal, the longitudinal axis of liquid crystal molecules is aligned vertically to the substrate. In contrast, at an initial stage of the VATN-LCD, liquid crystal molecules of the VATN-LCD are aligned similar to an alignment of the liquid crystal molecules of the TN-LCD to which voltage has been applied. In this state, if voltage is applied to the liquid crystal of the VATN-LCD, liquid crystal molecules of the VATN-LCD are twisted.
In an off-state of the VATN-LCD, liquid crystal molecules are vertically aligned and retardation of light does not occur if a viewing angle matches with an alignment direction of the liquid crystal molecules. Therefore, a superior contrast ratio may be achieved at the above viewing angle. However, if the viewing angle does not match with the alignment direction of the liquid crystal molecules, retardation of light may occur due to the birefringence of the liquid crystal, causing inferior contrast ratio. In particular, the contrast ratio is extremely lowered when the viewing angle forms an angle of 45° with respect to a transmission axis of a polarizing plate.
Liquid crystal having vertically aligned molecules has a characteristic identical to that of a C plate having a positive birefringence. Accordingly, the above birefringence of the liquid crystal can be compensated for by means of a compensation film made from a C plate having a negative birefringence. Preferably, an optimum retardation value of the compensation film made from C plate is substantially identical to a retardation value created due to the birefringence of the liquid crystal cell.
U.S. Pat. No. 4,889,412 discloses a technique regarding a negative (−) C plate. A main function of the negative C plate is to compensate for a black state of a VA-LCD at a no-voltage state or a low-voltage state. However, a conventional negative C plate represents a limited phase difference wavelength dispersion ratio value
  (            R      450              R      550        )so that the conventional negative C plate cannot properly compensate for the black state and color variation of RGB at various viewing angles.
For instance, a wavelength dispersion ratio value of an LCD panel has a normal wavelength dispersion characteristic, and a C plate having a super-high wavelength dispersion characteristic is required in order to compensate for the black state and color variation of RGB created in the LCD panel. However, it is difficult to achieve such a super-high wavelength dispersion characteristic by using one conventional phase difference film.
In the black state, the VA-LCD represents characteristics identical to those of a positive (+) C plate. In order to completely compensate for the positive C plate by using the negative C plate, the negative and positive C plates must have the same wavelength dispersion characteristic. Liquid crystal used for an LCD has a normal wavelength dispersion characteristic, and the wavelength dispersion ratio value
  (            R      450              R      550        )thereof is large. Thus, it is difficult to fabricate a phase difference film adaptable for the wavelength dispersion characteristic of liquid crystal of the VA-LCD by using the conventional negative C plate having the limited phase difference wavelength dispersion ratio value.
The positive C plate has been used for minimizing color variation of a CLC polarizing plate (for a brightness enhancement) at various viewing angles. However, the positive C plate also has a limited phase difference wavelength dispersion ratio value
      (                  R        450                    R        550              )    ,so the positive C plate represents problems identical to those of the negative C plate.
For instance, the positive C plate must have a super-high wavelength dispersion characteristic in order to minimize color variation of the CLC polarizing plate, but the conventional C plate phase difference film cannot provide the positive C plate having the super-high wavelength dispersion characteristic.
In a case of the CLC polarizing plate, it is necessary to provide the positive C plate representing the wavelength dispersion characteristic identical to that of the CLC. However, since the wavelength dispersion ratio value of the CLC is very large, it is difficult to fabricate the phase difference film adaptable for the wavelength dispersion characteristic of the CLC by using the conventional positive C plate.